Because of its electrical and heat conducting properties, copper has many important uses in the form of wire, sheet, etc. However, pure copper has relatively weak tensile strength. One promising approach to improving the strength of copper is to mix it with a non-alloying ductile phase and mechanically reduce it in size. Such multi-phase copper alloy mixtures have been referred to as "in-situ" composites or deformation processed composites. The alloying metal is present as an array of elongated particles.
It has been demonstrated that quite high strength copper-X alloys can be produced by alloying copper with elements where X is an element such as niobium and vanadium, or other refractory metal. See Harbison and Bevk technical article entitled "Superconducting and mechanical properties of in situ formed multifilamentary Cu-Nb.sub.3 Sn composites", American Institute of Physics (1977); and Bevk, et al. technical article entitled "Mechanical Properties of Cu-Based Composites With In-Situ Formed Ultrafine Filaments", IN SITU COMPOSITES IV, Elsevier Publishing Co., Inc. (1982). High strength sheets or wires may be fabricated by a casting and mechanical reduction process or by powder processing and mechanical reduction. The casting is first produced as a microstructure of X dendrites in a Cu matrix, and the alloy can then be mechanically reduced by either rolling or drawing operations. This kind of mechanically worked copper composite alloy is described by Downing, et al. (1987), and Verhoeven, et al. U.S. Pat. No. 4,378,330. In the powder processing technique powders of Cu and X are mixed and compacted followed by mechanical reduction. See Trybus, et al. (1988) Processing and Properties for Powder Metallurgy Composites, pp. 97-105, Ed. P. Kumar, K. Vedula and A. Ritter, The Metallurgy Society AIME.
These Cu-X deformation processed alloys are quite ductile and may be mechanically reduced to very large drawing strains without breakage. Mechanical reduction, such as by drawing, extrusion, or rolling, converts the X particles into elongated filaments, which serve to reinforce and greatly increase the strength of the formed wire, sheet, or other configuration.
It is known that alloys of copper with refractory metals such as chromium have their mechanical properties improved by a heat treatment, which is sometimes referred to as "age hardening". Refractory metals are slightly soluble in the copper matrix. By relatively low temperature heat treatment, refractory metal in solid solution can be caused to separate in the form of minute particles which collect throughout the volume of the Cu matrix. This is in addition to any large refractory metal particles which may be present in the Cu matrix if the composition of X is above around 0.5%. Age hardening can improve the mechanical strength of such alloys.
Temperatures most effective for age hardening of alloys are from about 350.degree. to 550.degree. C., but broader temperature ranges have been disclosed in several patents. These alloys are referred to herein as copper-refractory metal composite alloys (Cu-RF) alloys. U.S. Pat. Nos. 2,025,662 and 2,033,709 of Hensel, et al. disclose copper-chromium alloys containing 0.8 to 2.54, or 0.01 to 5% cbromium. As described in these patents, following casting of the alloys and while they are still in a molten state, some chromium tends to separate, rising to the top of the castings by density segregation, chromium being lighter than copper. Rapid cooling is proposed to minimize such segregation. After the castings have been formed and solidified, a heat treatment at 250.degree. to 600.degree. C. is described. This reheating or aging step is said to cause precipitation of the dissolved chromium, which becomes distributed in extremely small particles, and produces age hardening.
British Pat. No. 582,236 also relates to the preparation of copper-chromium alloys. It is stated that the chromium content may range from 1 to 35%. After casting of the alloy, it is subjected to a heat treatment, described as an annealing treatment carried out at a temperature between 400.degree. and 7500C. for one-half hour and up to 8 hours, to produce age hardening.
Australian Pat. No. 252,357 relates to copper-based alloys containing chromium and/or zirconium. It is stated that an age-hardening treatment has been proposed in which similar alloys are heated to temperatures of 700.degree. to 1000.degree. C. For the preparation of an alloy containing both chromium and zirconium, it is proposed to carry out a heat treatment from above 1000.degree. C. up to the solidus temperature of the alloy, followed by rapid quenching and thereafter aging of the alloy at a temperature of from 300.degree. to 5000.degree. C. to produce age hardening.
The above-described patent references do not specify the use of an inert atmosphere in carrying out the heat treatments described therein. Further, the heat treatments are not described as modifying any surface properties of the alloys. Their function is to improve mechanical properties by age hardening.